Refactor load_preferred_plugins

skimage/io/_plugins/null_plugin.ini

@@ -1,3 +1,3 @@
 [null]
 description = Default plugin that does nothing
-provides = imshow, imread, _app_show
+provides = imshow, imread, imsave, _app_show

skimage/io/_plugins/null_plugin.py

@@ -1,4 +1,4 @@
-__all__ = ['imshow', 'imread', '_app_show']
+__all__ = ['imshow', 'imread', 'imsave', '_app_show']
 
 import warnings
 
@@ -17,4 +17,9 @@ def imshow(*args, **kwargs):
 def imread(*args, **kwargs):
     warnings.warn(RuntimeWarning(message))
 
+
+def imsave(*args, **kwargs):
+    warnings.warn(RuntimeWarning(message))
+
+
 _app_show = imshow

skimage/io/_plugins/plugin.py

@@ -25,9 +25,14 @@ plugin_module_name = {}
 plugin_meta_data = {}
 
 preferred_plugins = {
-    'io': ['matplotlib', 'pil', 'qt', 'freeimage', 'null']
+    'all': ['matplotlib', 'pil', 'qt', 'freeimage', 'null'],
+    # Use PIL as the default imread plugin, since matplotlib (1.2.x)
+    # is buggy (flips PNGs around, returns bytes as floats, etc.)
+    'imread': ['pil'],
+    'imread.tiff': ['tifffile'],
 }
 
+
 def _clear_plugins():
     """Clear the plugin state to the default, i.e., where no plugins are loaded
 
@@ -43,23 +48,24 @@ _clear_plugins()
 
 def _load_preferred_plugins():
     # Load preferred plugin for each io function.
-    io_funcs = ['imsave', 'imshow', 'imread_collection', 'imread']
-    for func in io_funcs:
-        for plugin in preferred_plugins['io']:
-            if plugin not in available_plugins:
-                continue
-            try:
-                use_plugin(plugin, kind=func)
-                break
-            except (ImportError, RuntimeError, OSError):
-                pass
+    io_types = ['imsave', 'imshow', 'imread_collection', 'imread']
+    for p_type in io_types:
+        _set_plugin(p_type, preferred_plugins['all'])
 
-    # Use PIL as the default imread plugin, since matplotlib (1.2.x)
-    # is buggy (flips PNGs around, returns bytes as floats, etc.)
-    try:
-        use_plugin('pil', 'imread')
-    except ImportError:
-        pass
+    plugin_types = (p for p in preferred_plugins.keys() if p != 'all')
+    for p_type in plugin_types:
+        _set_plugin(p_type, preferred_plugins[p_type])
+
+
+def _set_plugin(plugin_type, plugin_list):
+    for plugin in plugin_list:
+        if plugin not in available_plugins:
+            continue
+        try:
+            use_plugin(plugin, kind=plugin_type)
+            break
+        except (ImportError, RuntimeError, OSError):
+            pass
 
 
 def reset_plugins():

skimage/io/tests/test_plugin.py

@@ -1,3 +1,5 @@
+from contextlib import contextmanager
+
 from numpy.testing import assert_equal, raises
 
 from skimage import io
@@ -27,6 +29,16 @@ def teardown_module():
     io.reset_plugins()
 
 
+@contextmanager
+def protect_preferred_plugins():
+    """Contexts where `preferred_plugins` can be modified w/o side-effects."""
+    preferred_plugins = plugin.preferred_plugins.copy()
+    try:
+        yield
+    finally:
+        plugin.preferred_plugins = preferred_plugins
+
+
 def test_read():
     io.imread('test.png', as_grey=True, dtype='i4', plugin='test')
 
@@ -93,12 +105,29 @@ def test_available():
     assert 'test' in io.find_available_plugins(loaded=True)
 
 
-def test_load_preferred_plugins():
+def test_load_preferred_plugins_all():
     from skimage.io._plugins import null_plugin
-    plugin.preferred_plugins['io'] = ['null']
-    plugin.reset_plugins()
-    plug, func = plugin.plugin_store['imshow'][0]
-    assert func == null_plugin.imshow
+
+    with protect_preferred_plugins():
+        plugin.preferred_plugins = {'all': ['null']}
+        plugin.reset_plugins()
+
+        for plugin_type in ('imread', 'imsave', 'imshow'):
+            plug, func = plugin.plugin_store[plugin_type][0]
+            assert func == getattr(null_plugin, plugin_type)
+
+
+def test_load_preferred_plugins_imread():
+    from skimage.io._plugins import null_plugin
+
+    with protect_preferred_plugins():
+        plugin.preferred_plugins['imread'] = ['null']
+        plugin.reset_plugins()
+
+        plug, func = plugin.plugin_store['imread'][0]
+        assert func == null_plugin.imread
+        plug, func = plugin.plugin_store['imshow'][0]
+        assert func != null_plugin.imshow
 
 
 if __name__ == "__main__":